Mike,
Do you mean a probe? for when you are digging after
you have detected the item with your detector.
If so I think Uniprobe is good prices vary on what one.
Periscope is the best, but no longer made and hard to find,
Ebay prices have been around $500-600, that is about what
they cost new.
Also you can get a Sunray probe for Minelab and Garrett's
and maybe some others that run inline with the detector.